Thermophilic bacteria are organisms which are capable of growth at elevated temperatures. Unlike the mesophiles, which grow best at temperatures in the range of 25-40.degree. C., or psychrophiles, which grow best at temperatures in the range of 15-20.degree. C., thermophiles grow best at temperatures greater than 50.degree. C. Indeed, some thermophiles grow best at 65-75.degree. C., and some of the hyperthermophiles grow at temperatures up to 130.degree. C. (See e.g, J. G. Black, Microbiology Principles and Applications, 2d edition, Prentice Hall, N.J., [1993] p. 145-146).
The thermophilic bacteria encompass a wide variety of genera and species. There are thermophilic representatives included within the phototrophic bacteria (i.e., the purple bacteria, green bacteria, and cyanobacteria), eubacteria (i.e., Bacillus, Clostridium, Thiobacillus, Desulfotomaculum, Thermus, lactic acid bacteria, actinomycetes, spirochetes, and numerous other genera), and the archaebacteria (i.e., Pyrococcus, Thermococcus, Thermoplasma, Thermotoga, Sulfolobus, and the methanogens). There are aerobic, as well as anaerobic thermophilic organisms. Thus, the environments in which thermophiles may be isolated vary greatly, although all of these organisms are always isolated from areas associated with high temperatures. Natural geothermal habitats have a worldwide distribution and are primarily associated with tectonically active zones where major movements of the earth's crust occur. Thermophilic bacteria have been isolated from all of the various geothermal habitats, including boiling springs with neutral pH ranges, sulfur-rich acidic springs, and deep-sea vents. For all of these organisms, it appears that the organisms present in these geothermal habitats are optimally adapted to the temperatures at which they are living (T. D. Brock, "Introduction: An overview of the thermophiles," in T. D. Brock (ed.), Thermophiles. General, Molecular and Applied Microbiology, John Wiley & Sons, New York [1986], pp. 1-16). Basic as well as applied research on thermophiles has provided some insight into the physiology of these organisms, as well as promise for use of these organisms in industry and biotechnology.